Almost 80% of the Earth's fresh water is locked up in the
cryosphere, i.e. snow, ice and permafrost. The cryosphere plays an
important role in moderating the global climate – and as
such, the consequences of receding ice cover due to global warming
are far reaching and complex.

Concept Science

Due to its high albedo, ice masses directly affect the global
energy budget by reflecting about 80% of incident sunlight back out
to space. Thus, once formed, ice tends to be maintained. However,
if ice cover were to decrease, less solar radiation would be
reflected away from the surface of the Earth – causing the
ice to absorb more heat and consequently melt faster still.

Around the North Pole, an area of sea ice the size of Europe
melts away every summer and then freezes again during the winter.
The thickness of Arctic sea ice plays a central role in polar
climate as it moderates heat exchange by insulating the ocean from
the cold polar atmosphere.

A decrease in sea ice could disturb ocean circulation in
patterns in the North Atlantic As sea ice forms, the salinity and
therefore the density of the upper ocean increase. The density
increase causes the surface waters to sink – in essence
acting as a pump, driving cold, deep ocean currents from the polar
regions towards the Equator. A reduction in Arctic sea ice could
significantly disrupt the Gulf Stream which transports warm surface
waters northwards from the Gulf of Mexico to the sub-polar waters
east of Greenland. It is thanks to the Gulf Stream that north-west
Europe currently enjoys annual temperatures of about 9° C
higher than average for the latitude.

As well as influencing how much sunlight is reflected back to
space, continental ice has an impact on sea level. The large ice
sheets covering Antarctica and Greenland amount to about 28 million
km3, which means that sea level is about 65 m lower than it would
be if these ice sheets didn't exist. Whilst evidence suggests that
these ice sheets are relatively stable, there are indications that
rapid changes are occurring around their margins.

CryoSat-2's icy mission is dedicated to monitoring very precise
changes in the elevation and thickness of polar ice sheets and
floating sea ice over a 3-year period. The observations that
CryoSat-2 makes will determine whether or not our ice masses are
thinning due to global warming.

Sea ice

Sea ice typically covers up to 15 million square kilometres of the
Arctic Ocean, and up to 19 million square kilometres of the
Southern Ocean around Antarctica, during their respective winter
seasons. This seasonal cycle of sea ice is one of the most dynamic
components of the Earth's climate system.

Floating sea ice

Unlike icebergs, which are chunks of ice that have broken off the
edges of ice-shelves or fronts of glaciers where they reach the
sea, sea ice is seawater that has frozen. It contains little salt
as most of it is rejected as it forms. Sea ice covers the Arctic
Ocean more or less permanently above the latitude of about
75°N. This permanent ice cap is composed of pack ice, which is
kept in continuous motion by the wind, tides and ocean
currents.

Regional sea-ice models have been successfully developed over
the last decades. However, given the impact that sea ice has on the
climate, it is essential to acquire more comprehensive data on sea
ice thickness to improve sea ice models for their implementation in
general climate studies.

To further our understanding of the impact that sea ice has on
climate and to ascertain whether there is currently a trend towards
reduced sea ice cover, CryoSat-2 will provide new and authoritative
data on fluctuations in Arctic and Antarctic sea ice.

Ice sheets

Sea levels rose by 18 cm during the last century alone,
and there are major concerns about what a warmer climate means for
sea-level rise for this century.

An obvious source for this extra water is from the melting of
ice sheets and glaciers overlying land. The ice sheets that blanket
Antarctica and Greenland are up to around four kilometres thick,
and it is the melting of these large ice masses that have the
potential to cause a significant rise in global sea level.

Glacier calving

However, observations from ERS indicate that the great central
plateaux of the Antarctic and Greenland ice sheets are relatively
stable (Wingham et al., 1998). Nevertheless, there are indications
that changes are occurring at the margins of the ice sheets and it
is these apparent changes that need to be quantified. Also, dotted
around the globe, smaller glaciers play a significant role in local
hydrology, sea level rise and regional climate. However, evidence
from a variety of sources suggests that the majority of the world's
glaciers are currently retreating.

The improvement in resolution of the CryoSat-2 radar over that
of its pulse-limited predecessors, coupled with its interferometric
capability, will make spatially and temporally continuous
measurements of the ice-sheet margins and smaller ice masses
possible for the first time.

Polar climate

Sea-ice thickness plays a central role in climate processes. Ice
extent and thickness have important stabilising effects on world
climate, insulating large areas of the oceans from solar radiation
in the summer and preventing heat loss to the atmosphere in the
winter.

The effects of a warming climate are expected to be felt more
strongly at the poles, and particularly in the Arctic. However,
current global ocean-ice-atmosphere models are not yet able to
accurately reproduce observed sea-ice coverage.

Disappearing Arctic sea ice predicted under CO2
doubling

Arbitrary flux corrections in these models, which essentially
hold the sea surface temperature at freezing in regions where there
is sea ice, make the calculation of the effect of 'perturbations'
such as CO2 –induced warming questionable (Gates et al,
1996).

The physics of sea ice in global climate models is, at the
moment, oversimplified. With the recognition that variations in
thermohaline circulation may have important consequences for
poleward-bound heat transport – the next years will see more
complete sea-ice physics within global ocean-atmosphere models.
These developments demand more information on sea-ice extent and
volume changes – which will be provided by the CryoSat-2
mission.